1. Field of the Invention
The present invention relates to a multiple-frequency signalling system including multiple-frequency sender means which are coupled to multiple-frequency receiver means via a transmission path. The sender is adapted to generate a plurality of alternating signals having different frequencies. The receiver means includes an evaluating circuit to evaluate the presence of a predetermined number of said alternating signals received by said receiver means. Said receiver means further including a plurality of channel paths each including a filter tuned to a frequency of said alternating signals and inhibiting means which render said channel paths ineffective upon spurious signals being applied to them. The channel path being rendered effective when simultaneously said channel path receives a signal having a frequency equal to that to which the filter included in this channel path is tuned and said evaluating circuit evaluates a said predetermined number of alternating signals.
2. Description of Prior Art
Such a system is known from the U.S. Pat. No. 3,072,747. Therein digits are each transmitted in the form of a combination of two alternating signals out of a plurality of six such signals. The inhibiting means included in each channel path of this system comprise an additional filter or rejector circuit accepting said plurality of frequencies but having a rectified output which is in opposition to the rectified output of the filter included in this channel path and tuned to a particular one of this plurality of frequencies. In this known system, when spurious signals are received by the receiver means the rejector circuit of each channel path rectified output of this circuit overrides that of the tuned filter included in this path and inhibits the latter. While the use of such rejector circuits enables the elimination of the effect of spurious signals, it entails the use of supplementary band-stop filters to receive the above combinations of frequency signals. Indeed, without such band-stop filters a reception of a combination of for instance two frequency signals having frequencies F1 and F2 will cause an erroneous operation of the receiver means since although the frequency signal with frequency F1 is correctly received by the filter tuned to this frequency, the rejector circuit included in the same channel path as this filter will treat the signal of frequency F2 as a spurious signal and consequently render this channel path ineffective due to the rectified output of the rejector circuit overriding that of the tuned filter. Such an erroneous operation is however eliminated by the use of a band-stop filter tuned to the frequency F2 preventing the frequency signal having this frequency F2 from reaching the rejector circuit. However, it is not necessary to use a band-stop filter per combination since these combinations of two frequency signals are grouped in such a way that each group has a particular frequency in common, it being thus possible to use one band-stop filter per group of combinations. This band-stop filter is tuned to the common frequency of the group and precedes a group of channel paths each including a filter tuned to a distinct non-common frequency of the combinations of that group.
Table 2 of the above mentioned U.S. patent shows the grouping of frequencies in four groups. Each of these groups contains a common or band-stop filter frequency F2; F3; F4; F5 and noncommon or acceptor frequencies F1, F3, F4, F5; F1, F5, F6; F1 F3; F1, F4, F6 respectively. Therefrom it follows that the number of tuned filters and consequently the number of channel paths is much larger, more particularly twice as large as the number (6) of acceptor frequencies. It can easily be seen that this large number of channel paths is due to the introduction of rejector circuits and the accompanying band-stop filters. Indeed, it is obvious that in order to be able to receive, for instance, the combinations of frequency signals F1, F1 and F3, F1 it necessitates the presence of at least two filters tuned to the frequency F1. As mentioned above, each of the twelve channel paths moreover includes an additional filter or rejector circuit, while each of the four groups of tuned filters is preceded by a band-stop filter.